1. Field of the Invention
This invention relates generally to telecommunications, and, more particularly, to a method and apparatus for handling Plain Old Telephone System (POTS) and x-digital subscriber line (xDSL) signals.
2. Description of the Related Art
In communications systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire, bi-directional communication channel. A line card generally connects the subscriber station to the central switching office. The functions of the line card range from supplying talk battery to performing impedance matching to handling ringing signals, voice signals, and testing signals.
Equipment connected to telephone lines generally have a standardized impedance. To drive a signal on a subscriber line while minimizing signal reflection from the far end of the subscriber line and maximizing the signal power coming out the far end, it is desirable to match the standardized impedance. This impedance is typically symbolized as ZLOOP, which may be a function of frequency and decreases as frequency increases. For POTS lines, the value of ZLOOP is determined by individual telephone authorities in various countries and, although somewhat variable, is in the range of 600–900 ohms and may or may not include some type of capacitive element. The extent to which a signal driver is matched to the standardized impedance in these systems is measured with a parameter known as “return-loss.” Perfect matching will have an infinite return-loss. This indicates that none of the incident signal transmitted from a source with the standard impedance is reflected back to the source.
In an electronic Plain Old Telephone System (POTS), the impedance matching function has generally been performed by line cards using a variety of well-known impedance matching filter loops. The function of the impedance matching filter loop in POTS-only applications is to take the input signal, modify it through a programmable gain and phase element, and feed it back to the output so that the input signal sees a different response than it would without the presence of the impedance matching filter. The above-described impedance matching process is generally effective in accomplishing the intended purpose, at least as it pertains to a POTS-only system.
The Plain Old Telephone System, designed primarily for voice communication, provides an inadequate data transmission rate for many modem applications. To meet the demand for high-speed communication, designers have sought innovative and cost-effective solutions that would take advantage of the existing network infrastructure. Several technological solutions proposed in the telecommunications industry use the existing network of telephone wires. A promising one of these technologies is the xDSL (digital subscriber line) technology.
xDSL is making the existing network of subscriber lines more robust and versatile. Once considered virtually unusable for broadband communications, an ordinary twisted pair equipped with DSL interfaces can transmit video, television, and very high-speed data. The fact that more than six hundred million subscriber lines exist around the world is a compelling reason for these lines to be used as the primary transmission conduits for at least several more decades. Because DSL utilizes telephone wiring already installed in virtually every home and business in the world, it has been embraced by many as one of the more promising and viable options.
There are now at least three popular versions of DSL technology, namely Asymmetrical Digital Subscriber Line (ADSL), Very High-Speed Digital Subscriber Line (VDSL), and Symmetric Digital Subscriber Line (SDSL). Although each technology is generally directed at different types of users, they all share certain characteristics. For example, DSL systems utilize the existing, ubiquitous telephone wiring infrastructure, deliver greater bandwidth, and operate by employing special digital signal processing. Because the aforementioned technologies are well known in the art, they will not be described in detail herein.
DSL and Plain Old Telephone System technologies can co-exist in one line (e.g., also referred to as a “subscriber line”). Traditional analog voice band interfaces use the same frequency band, 0–4 Kilohertz (KHz), as telephone service, thereby preventing concurrent voice and data use. A DSL interface, on the other hand, operates at frequencies above the voice channels, from 25 KHz to 1.1 Megahertz (MHz). Standards for certain derivatives of DSL are still in definition, and, therefore, are subject to change. Thus, a single DSL line is capable of offering simultaneous channels for voice and data. It should be noted that the standards for certain derivatives of ADSL are still in definition as of this writing, and therefore are subject to change.
DSL systems use digital signal processing (DSP) to increase throughput and signal quality through common copper telephone wire. It provides a downstream data transfer rate from the DSL Point-of-Presence (POP) to the subscriber location at speeds of up to 1.5 megabits per second (MBPS). The transfer rate of 1.5 MBPS, for instance, is fifty times faster than a conventional 28.8 kilobits per second (KBPS).
Although DSL and POTS systems can co-exist on one line, the DSL traffic passing through the POTS circuitry impairs the functionality of the impedance matching filter of the POTS circuitry. This is because decimators and analog-to-digital converters that are ordinarily utilized in a POTS-only system cannot process the high frequencies of the data band, thus causing the performance of the POTS impedance matching filter to degrade. Additionally, the impedance required at data frequencies is much lower than at voice frequencies.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.